In recent years, there have been major advances in the design of aircraft structures. The most notable is the use of composite materials to replace the protective metal skins on the aircraft's wings. A design problem occurs with the use of composite materials, the protection from electromagnetic (EMI) and radio frequency interference (RFI) that the metal skins provide is lost. This interference will not allow conventional electronic sensors to be used with composite materials. In order to solve this unique design problem new sensor technologies have to be explored.
The new technology of fiber optic communications has created an emerging area of research in the use of optical fibers as sensing elements or in sensor systems. Fiber optic based sensors offer immunity to electromagnetic and radio frequency interference, increased sensitivity, larger bandwidths, high data transfer rates, compatibility with existing fiber telemetry, high melting point, and geometric versatility. These characteristics make fiber optic sensors an ideal choice to use in composite wing aircraft.
Existing fiber optic sensors require unique transmitter-receiver units configured for the individual sensor. An inexpensive transmitter-receiver unit that would allow the interchanging of sensors without redesigning the electronic circuitry is very desirable. Fiber optic cables allow the transmission and receiving of signals over the same cable by different modulation methods. However, it is desirable to select one modulation scheme to reduce the complexity of the design.
The two main modulation schemes used in sensor applications are phase and amplitude modulation. Phase modulation allows higher sensitivity, but the sensor configuration is complicated and requires the use of expensive lasers and single mode fibers. Amplitude modulation trades sensitivity for low cost and ease of configuration by using light emitting diodes and multimode fibers. A fiber optic based sensor using amplitude modulation would provide a simple, low cost solution to the interference problem inherent in the use of composite materials.
Development of optical fiber sensors started around 1977. Since then acoustic, magnetic, pressure, acceleration, temperature, gyro, displacement, fluid level, torque, current, strain and stress sensors are among the fiber optic sensors that have been developed. These were developed using either single or multimode fibers and incorporated many different modulation schemes.
Multimode fiber sensors have advanced very rapidly due to the low cost of the fiber, alignment systems, sources, detectors, and simpler configuration. There are temperature sensors based on the temperature-dependent optical absorption property of semiconductor materials, current transformers based on the magneto-optic Faraday effect, voltage transformers based on the electro-optic Pockels effect, and pressure sensors based on microbending losses just to name a few.
Displacement sensors using multimode fibers have been developed using microbending induced mode coupling of core to cladding modes, speckle dynamics, and Michelson interferometric methods. Adamovsky has researched a fiber optic displacement sensor with spatially separated signal and reference mirrors. This research involved using frequency domain referencing of the relative amplitudes of the signal and reference frequencies as a function of displacement. Adamovsky demonstrated the detection of the relative amplitudes of the signal and reference frequencies by analyzing different portions of the fourier signal spectrum. The limitation of this scheme is that the signal processing is significantly effected by non-repeatable fiber/fiber connector losses. These connector losses occur at bulkhead connectors which are inherant in any practical fiber optic sensing system.